Mirrors are made by the successive application of various metals, preferably silver and copper films of between 800 angstroms and 1,500 angstroms in thickness on suitable substrates of glass or plastics such as polycarbonates, by wet chemical plating processes or vacuum metallizing, followed by a paint coating application, usually known as a mirror backing.
Metallic layers are applied to glass substrates by one of three general methods, 1) electroplating, 2) chemical deposition, or 3) galvanic deposition. Galvanic deposition is the current method of choice and is now widely used. Usually a silver layer is applied and a copper layer deposited on the silver layer. The copper layer is important to provide good adherence of the backing to the silver layer. The present invention employs the galvanic deposition system.
It is known that reflective silver layers on mirrors, even when protected by "backing" coatings are extremely sensitive to corrosive decomposition when exposed to moisture, contaminants in the atmosphere, salt, hydrogen sulfide, ammonia, and chlorides, which are present in domestic environments. It is also known that zinc phosphate or zinc salts of dicarboxylic acids, commonly applied as corrosion inhibitors in protective primers on steel or aluminum, contrary to expectations, do not work well in mirror backing coatings and actually exhibit corrosive action on the silver layer.
Various mirror backing coatings have been proposed by the prior art, including coatings which comprise 20 mixtures of an organic resin and pigment. For example, U.S. Pat. No. 4,707,405 to Evans et al, discloses a mirror backing coating which comprises an organic film forming resin and a cyanamide salt of a non-lead metal. The cyanamide salt is said to be a salt of group IIA or group IIB metal such as calcium, zinc or magnesium, with the preferred salt being calcium cyanamide. It is believed that the calcium and magnesium cyanamides have not been successful because both are water soluble, highly alkaline "non-pigment" products which interfere in detrimental fashion with curing processes of coating systems organic material. In addition, they exhibit only marginal corrosion preventive activity on silver, which is expected considering that typical technical grade calcium cyanamides contain considerable amounts (e.g., 1% CaS.sub.2) of soluble sulfide species as impurities. Therefore, the zinc cyanamides known heretofore have not gained commercial acceptance.
In parent application Ser. Nos. 07/754,898 and 07/902,206, an invention is set forth which provides superior anti-corrosion mirror backings using a non-lead pigment.
The present invention provides improved paint and coating formulations for mirror backings which have excellent long term corrosion preventive protection of the mirror reflective silver layer utilizing a novel resin formulation.